1. Field of the Invention
This invention relates to a vacuum booster device which is mainly used to operate a master cylinder for hydraulic brakes, clutches or the like for automobiles by boosting negative pressure, and more particularly to a booster device of the type having: a booster shell; a booster piston housed in the booster shell to be movable forward and backward; a diaphragm which includes an inner peripheral bead fitted onto the rear surface of the booster piston, an outer peripheral bead fitted onto the peripheral wall of the booster shell and a pressure receiving portion superposed on the rear surface of the booster piston, and which is adapted to divide in cooperation with the booster piston the interior of the booster shell into a front-side first working chamber communicating with a vacuum source and a rear-side second working chamber; an input rod supported by the booster piston for advancing and retreating movements; and a controlling valve operatively connected with the input rod to selectively communicate the second working chamber with the first working chamber or the atmosphere.
2. Description of the Prior Art
In a vacuum booster device of the mentioned type, when the booster piston is manually operated to actuate the master cylinder in a state where the vacuum source is not operated, i.e., negative pressure is not stored in the first working chamber, air within the first working chamber is compressed due to advancing movement of the booster piston. Thus, advance of the booster piston is obstructed to some degree by virtue of resistance caused when the air is discharged to the vacuum source. As a result, the device tends to lose smoothness and lightness in its manual operation.